Many of these organisms have left their remains as fossils in sedimentary rocks.
The method of reading the order is called stratigraphy (layers of rock are called strata).
Relative dating does not provide actual numerical dates for the rocks.
This process requires much more sophisticated chemical analysis and, although other processes have been developed, often utilizes the decay rates of radioactive isotopes to determine the age of a given material.
Using this process geologists are able to assign actual ages with known degrees of error to specific geologic events.
For example, microscopic dinoflagellates have been studied and dated in great detail around the world.
Correlation with them has helped geologists date many New Zealand rocks, including those containing dinosaurs.
The activity offers literacy opportunities as well as practice using the science capability 'Interpret representations'.
The most obvious feature of sedimentary rock is its layering.
For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic.
Different species of ammonites lived at different times within the Mesozoic, so identifying a fossil species can help narrow down when a rock was formed.